Benchmark framework by let them doing the same work.

[fakeshadow]

There are some obvious problem with the bench.

1. actix-web and warp return a String while others return a &'static str. This means they have additional allocation for generating payload while others don't.

2. hyper being low level does not set content-type header. And I believe all other tests do set it to "plain/text" or "text/plain; charset=utf-8". So it should set one to be on par.

3. warp by the test method mentioned in README.md does not generate meaningful response because it can not find the route you defined. This makes it return a 404 response that has no payload so it bypass the response body entirely. This is mentioned in issue Rocket v5 (async) bencmark #2.

[rousan]

@fakeshadow, can you please send PR?